Several prior art sound reproducing systems have included the loudspeaker in a feedback loop for reducing loudspeaker distortion, and for allowing use of smaller loudspeakers and smaller loudspeaker enclosures. Such prior art systems, especially those which include means for magnetically sensing the axial motion of the associated loudspeaker cone, have considered neither the detrimental effects due to electrical interference from the main electromagnetic loudspeaker structure, nor the proper frequency shaping of the motional feedback signal to cause the loudspeaker to respond linearly to the input source signal. Such prior art sound reproducing systems are cited in U.S. Pat. No. 3,798,374 entitled "Sound Reproducing System Utilizing Motional Feedback" issued on Mar. 19, 1974 to Stanley T. Meyers. This prior art system uses passive RC equalizers and filters which result in phase shifts which vary from near zero at high frequencies to near 90.degree. at low frequencies thereby failing to correct distortion and failing to provide a wide band uniform frequency response.
A more recent sound reproducing system is disclosed in U.S. Pat. No. 4,550,430 entitled "Sound Reproducing System Utilizing Motional Feedback and an Improved Integrated Magnetic Structure" issued Oct. 29, 1985 to Stanley T. Meyers which discloses a sound reproducing system utilizing motional feedback and an integrated magnetic structure for reducing loudspeaker distortion; for reducing acoustic coupling between the radiated sound energy output and the cone motion sensing structure thereby reducing unwanted feedback signals; and for reducing obstruction of the radiated sound energy output by the cone motion structure thereby minimizing alteration of the radiated tonal quality. The loudspeaker includes a cone, a frame, and flexible webs. The main electromagnetic structure includes a rear cylindrical iron pole piece, an annular cylindrical permanent magnet, an inner annular cylindrical iron pole piece, a main voice coil bobbin, a main voice coil, and a front annular cylindrical iron pole piece. The cone motion sensing structure includes a front annular copper disc, a rear cylindrical non-magnetic support member, an annular cylindrical copper sleeve, a cylindrical iron rod, a feedback sensing coil, a feedback sensing coil bobbin, and a feedback sensing coil bobbin support member. The associated circuitry includes a passive stability control network, a passive velocity equalizer, and a passive frequency equalizer which do not provide a wide band uniform frequency response and uniform phase shift. Thus, the system disclosed in the Meyers U.S. Pat. No. 4,550,430 has the same above-noted shortcomings and drawbacks as the system disclosed in the Meyers U.S. Pat. No. 3,798,374.
Early sound reproduction systems of general background interest are disclosed in U.S. Pat. No. 3,530,224 entitled "Motional Feedback Amplifier Systems" issued Sep. 22, 1970 to M. G. Reiffin, U.S. Pat. No. 3,118,972 entitled "Acoustic Apparatus" issued Jan. 21, 1964 to S. Walczak and U.S. Pat. No. 2,948,778 entitled "Sound Reproducing Means" issued Aug. 9, 1960 to W. W. Clements.